Flat panel displays, such as a liquid crystal display and a plasma display, are applied in a wide range from mobile devices, such as a portable telephone, to large equipments, such as a public display. Most of those displays are developed with a focus on a wide viewing angle, high brightness, and high image quality, being required to display an image with clearness and easy to be viewed at any angles.
Meanwhile, some contents shown on displays include confidential information and private data which are not supposed to be viewed in public. Therefore, it is an important issue that a displayed content is protected from general public viewing in a ubiquitous environment that has been developed today along with development of information devices. Even in an office, there is a case where some confidential information, which needs to be protected against view by someone walking behind a seat, is handled.
Some portable telephones include a display with an optical masking shield (a louver) so that a displayed content can be viewed only from a specific direction. Even in such a case, the displayed content can be viewed by someone just behind a person handling it, and security protection is not sufficient.
As a technique to solve the above mentioned problem, there is an “image display device” disclosed in Patent Document 1. The image display device presents an image (hereinafter, referred to as a secret image) to a viewer wearing glasses with an image selecting function, and also presents another image (hereinafter, referred to as a public image) to other viewers.
Specifically, FIG. 11 shows an image display device, in which input image signals 11 are stored in an image data storage memory 12 with a capacity of signals for one frame, controlled by a frame signal 13, then image data in the image data storage memory 12 are read out at twice the speed of frame period, and signals read out primarily are compressed into a half size to become a first image signal 14 and are inputted into a synthetic circuit 15. The secondly read-out image signals have their chroma and luminance converted to become second image signals 17 and inputted into the synthetic circuit 15. Therefore, an image display 18 shows the first image signals 14 and the second image signals 17 alternately. Meanwhile, the frame signal 13 drives a shutter of glasses 21 using a glasses shutter timing generation circuit 19 so that a viewer cannot view an image depending on the second image signals 17. According to such a configuration and operation, those who without glasses view a gray image or a third image (a public image) which is a synthetic image of the first image signals 14 and the second image signals 17, and which has no correlation with the first image signals; on the other hand, those who wear glasses can view a desired image (a secret image) based on the first image signals.
Further, as another technique to solve the aforementioned problem, there is a “Secure method for providing privately viewable data in a publicly viewable display” disclosed in Patent Document 2. The method for providing data capable of being viewed privately, disclosed in Patent Document 2, allows only a permitted user (a viewer) to decipher a private image (a secret image) on a display, at the same time, that allows an unpermitted user to simply view a random pattern, an image difficult to be deciphered, or may be a screen saver image (a public image). In order to promote the object above, an image processing technique including a data hiding pattern and an alternation pattern is synchronized with a display including an image made by an image processing technology, that is, for example, combined with a wearable device such as active glasses. Finally, by a known ability of visual system of human for making a dissimilar image be merged into a single image, a performance can be complete in which data capable of being viewed privately can be presented on a display viewed publicly.    Patent Document 1: Japanese Patent Application Laid-open No. 63-312788    Patent Document 2: Japanese Patent Application Laid-open No. 2001-255844